4 Additional Types of Control Charts
4-1. X-Bar and S Charts
X-bar and R-charts are very popular and widely used. Historically, the reason that the range was used as a measure of variability is that it is easy to compute by hand. It provides good statistical information for small sample sizes (less than 8). The standard deviation, while more difficult to compute, is a better measure of the variability of data, especially for large sample sizes. This is because it uses all of the data, whereas the range uses only two observations. An alternative to the R-chart is the standard deviation chart, or s-chart.
When the standard deviation is easy to calculate, such as when the data are recorded on a computer or if a calculator is available that has ...